Front-surface reflection effect

A few points might be made about the assumption of isotropic scatter. If there is specular reflection in the scatter, there may be preferential directions of travel through the sample, and the assumption of diffuse radiation will be violated. Assumption 3 points out that the effect of front-surface reflection is ignored in their treatment. This assumption has often been interpreted as meaning that forward and backward scatter from a particle are assumed to be equal. As stated above, related to Equation (3.24) and Equation (3.25), the assumption of isotropic scatter of this kind is also built into their treatment. 

Because the Raman effect is a scattering process, it does not have the problems associated with the requirement of light transmission. Raman front-surface reflection allows the examination of a sample of any size or shape. If the sample can be hit with the laser beam, a Raman spectrum can usually be obtained. 

Raman Spectroscopy. As mentioned earlier, the Raman effect is an emission phenomenon, which means that front-surface sampling is possible. An irregularly shaped solid may be used in the spectrometer without processing it to make it flat, as required for internal reflection, or without processing it to a film or powder for transmission IR. Another sampling convenience is that Raman is more easily applicable to water solutions than IR. 

The angle of incidence of radiation on the thin-film coating has one major effect on the reflectivity, because the path difference of the front and rear surface reflection from any layer is a function of angle. The change in path difference results in a change of phase difference between the two interfering reflections, and thus perfect destructive interference is not any longer maintained. 

Spectral data must be examined for aborations caused by physical properties, such as surface flatness and smoothness, that influence flop angle, and specular and diffuse reflectivity. The effects may be in a local wavelength region, at a particular angle of measurement, or a combination outside the calibration parameters. Important information about the physical state of filmed specimens includes thicknesses, hardness, and durability of substrates and film. Soft films, which are used as front-surface calibration standards, are often overcoated to prevent oxidation and to improve durability for cleaning purposes (29). The overcoat material s type and thickness can considerably alter the spectral curves and data by interference (Fig. 13), or it... 

While calculating the radiative processes rate the influence of reabsorption effects was taken into account. A reflection coefficient of 0.1 at the front surface and 1 at the back surface were assumed. 

Second, the effect of reflection loss at the windows of the cell has been neglected in the treatment described above. The refractive index, n, of most organic samples and windows is about 1.5, so that the reflectance of the front surface is about 4% (see Section 13.2.2). If Beer s law is to be applied accurately, the apparent absorbance caused by reflection loss (0.018 absorbance unit for windows with n = 1.5) should first be subtracted from the measured absorbance spectrum. 

We should note that interference fringes are sometimes seen in transflection spectra. When the film is smooth but the substrate is rough, the full intensity of the beam that has been reflected from the front surface of the film will be measured at the detector. However, only a fraction of the beam that has been diffusely reflected from the substrate will be focused on the detector element. In this case, the fringes are not completely canceled, and the baseline of the transflectance spectrum exhibits a sinusoidal modulation. This effect can often be seen in the transflectance spectrum... 

The loads from external near-surface burst explosions are based on hemispherical surface burst relationships. Peak pressure (P psi) and scaled. impulse Ci/W psi/lb ) are plotted vs. scaled distance (R/W ft/lb ). Roof and sidewall elements, side-on to the shock wave, see side-on loads (P and i ). The front wall, perpendicular to the shock wave, sees the much higher reflected shock wave loads (P and i ). An approximate triangular pressure-time relationship is shown in Figure 5a. The duration, T, is determined from the peak pressure and impulse by assuming a triangular load. Complete load calculations include dynamic loads on side-on elements, the effect of clearing times on reflected pressure durations, and load variations on structural elements due to their size and varying distance from the explosive source. 

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